(1) Field of the Invention
The present invention relates to an intramedullary nail with dynamic compression, and a system and method of use thereof.
(2) Description of the Related Art
To heal an injured bone properly, the bone must remain in stable position with the edges of the bone compressed against one another without motion. Intramedullary nails have been employed in orthopedic surgery for centuries to serve this purpose, as they are an excellent source of lasting axial and torsional stability, stiffness and rigidity. Intramedullary nailing has a long and interesting history that dates back to the 16th century. Küntscher developed modern intramedullary nailing techniques in Germany during the 1940s. The first transfixion interlocked nail was introduced in 1953, and intramedullary nailing has since become the standard of care for the treatment of fractures of long bone diaphyseal fractures such as those in the femoral shaft, tibia, humerus, and radius that require operative stabilization. Retrograde intramedullary nailing has also been used in the foot and ankle to perform arthrodesis of the tibia to the talus to the calcaneus, or the tibia to the calcaneus.
One objective of osteosynthetic implants is the anatomic reduction of the fracture. Another objective would be to minimize or eliminate interfragmentary motion. Still another objective involves increasing or maximizing blood supply to the fracture site by reducing or minimizing additional vascular damage. Sustained compressive therapy can also be osteoinductive, due to its piezoelectric effects on osteoblasts themselves. However, problems can arise when the bone fracture or fusion site is not sufficiently stabilized during the healing timeframe. Excessive interfragmentary motion results in the formation of fibrous, unmineralized scar tissue (resulting in a non-union or pseudo-arthrosis) versus the regeneration of bone. The unmineralized scar tissue is not load-supporting and skeletal function is lost. Compression prevents interfragmentary motion and stimulates the formation of bone. A sufficient blood supply must be maintained to support skeletal metabolism, bone regeneration, and remodeling of the fracture site.
The current standard of care includes osteosynthetic devices that are made of either stainless steel or titanium. The use of stainless steel or titanium in osteosynthetic devices has a long history and reasonable record of success. However, over time, the stainless steel and titanium fixation constructs (screws, plates and nails) do not maintain compression across the fracture fragments. This is due to the biological resorption that occurs across the fracture or fusion site during normal primary bone healing. The stiffness of these materials and the relative constructs initially serves well to maintain the healing bones in close compressed approximation to one another, but after re-absorption they actually serve to maintain the bones separated or distracted from one another. The reduction of compression of certain standard material plate and screw constructs is well known, has been studied, and is observed to be thirty-two percent (32%) over a two-week period. Intramedullary nails have shown a 90% reduction in compression with only one millimeter of resorption across the fusion site. As the necrotic surfaces of the fracture are resorbed, a non-load bearing gap develops between the fragments, thereby decreasing compression and increasing the risk of interfragmentary motion and scar tissue formation. Loss of compression is contrary to the objectives of fracture or fusion fixation in general, and osteosynthetic implants in particular. Improvements are needed which will maintain a compressive load across the fracture or fusion site over a longer period of healing, and in some cases assist in keeping the compressive load traverse to the line of the break.
The efficiency and effectiveness of intramedullary nailing can be considerably enhanced if the device provided continuous dynamic compression across the fracture or fusion site. This need for compression has been identified in the current art. However, many intramedullary nail systems include devices, both internal and external to the nail itself, which provide compression only at the time of insertion. The problem remains that during the healing process, re-absorption results in a gap across the healing site, and compression is lost.
Attempts have been made to allow dynamic compression with nails currently on the market and which are well known to the art. For example, in one attempt, proximal transfixion screws have been placed in slotted holes, which allow the distal segment of bone and the nail itself to slide proximally. But, the current nails are designed to allow this compression only after weight bearing has begun, employing the very ground reactive forces encountered during weight bearing itself to provide the necessary compression. Unfortunately, resorption occurs much earlier during the healing process than does weight bearing, especially in the many cases in which additional procedures such as mid-foot fusion, are performed simultaneously with rear-foot to ankle fusion. Therefore, in many cases, non-union has already occurred by the time weight bearing is allowed, i.e. the gap forms and fills in with fibrous tissue long before weight is born on the extremity, thereby preventing the formation of solid bone. Where the current art contains compression, it does not allow selection of compressive force and it couples compression with positioning of the nail, which can be detrimental to achieving proper positioning with the correct compression force desired.
The current art also describes a telescopic screw with an internal spring which is activated by turning the screw head and tightening of the screw. As the screw is inserted, the leading edge or threaded portion of the screw, which is the male member, advances, thereby loading the internal extension spring, thereby effectively producing compression across the fusion or fracture site. Unfortunately, multiple limitations are inherent in this design. First, the amount of compression is extremely variable and may be minimal, due to the fact that the spring is only loaded proportionate to the amount that the female end of the screw is extended. This is dependent on the amount of available space in the bone for screw insertion. Secondly, the fact that the screw length changes during screw insertion is problematic because with a fixed amount of bone available for screw insertion, the screw must be a precise and predictable length to ensure appropriate placement in the anatomy. If the female member of the screw encounters hard bone during insertion, the leading male member will prematurely extend, thereby altering the length of the screw. What is needed is a device with a predictable and reproducible length and a predictable and reproducible amount of compression.
The current art also describes a device which provides continuous compression through a hydraulic mechanism that is internal or external to the body and compresses the nail. This is problematic in that hydraulic fluid mechanisms are excessively complicated and not desirable for implantation. External mechanisms to provide compression of an intramedullary nail must violate the skin barrier and traverse to the bone, thereby presenting the likely possibility of superficial or deep infection.
Nitinol or memory metal nails have been described, but memory metal is also complicated in many cases, requiring the device to be delivered to the OR frozen, or heated with special machinery during implantation in order to activate the compression mechanism. Also, in order for a memory metal nail to compress, it would require that it also expand in thickness or diameter, which is not desirable in this application.
Other patents described in the prior art require lengthening of a screw to provide compression, including U.S. Pat. Nos. 4,959,064 and 6,656,184 disclose active compression mechanisms within a screw which utilize an extension spring. This is disadvantageous in that, as noted above, the amount of compression is extremely variable and may be minimal due to the fact that the compression is dependent on the amount of available space in the bone for screw insertion. A screw having a varying length that changes during screw insertion is also problematic because the screw should have a precise and predictable length to ensure appropriate placement in the anatomy, considering the fixed amount of bone available for screw insertion. The devices are also disadvantageous in that the screws only utilize extension springs to provide compression and cannot use different or multiple compression means simultaneously to achieve dynamic compression.
In addition to lack of continuous dynamic compression, a provision for rotational stability at the level of the fusion site is lacking with Intramedullary Nail systems currently described in the art. As previously mentioned, bones heal primarily when motion is minimized at the site of healing. Motion may occur through distraction, translation, bending or rotation. While translation and bending forces are controlled well with the current intramedullary nail systems, it is well known that rotation may occur, which may negatively impact bone healing. This rotation may occur at the fusion site because the current nails are round and the bones may rotate around the nail's axis. This problem can be addressed by employing a nail with a cross sectional shape which does not allow the bones to rotate around its axis. This shape is especially important at the level at or around the fusion sites, and in the areas where it traverses cortical or subchondral bone.
Therefore, what is needed in the art is a simple and reliable mechanism for achieving continuous dynamic compression with an intramedullary nail, which can be implanted within the nail itself, and avoids the use of hydraulic fluid, external mechanisms, weight bearing or memory metal. Specifically, an intramedullary nail that can actively engage a pin crossing the bone to provide continuous dynamic compression without weight bearing and can provide to the user a choice using of one or more compression mechanisms is needed. What is also needed is a device of a fixed size and length and will provide predictable and reproducible continuous dynamic compression across the fusion site from the time immediately following the surgical procedure throughout the healing process, including the six to twelve or more weeks spent without weight bearing, and can be easily adjusted to the compressive force that is desirable at the time of its installation. An intramedullary nail that includes a shape that does not allow rotation of the bone around the nail's axis is also needed.
With these goals in mind, the inventor has created an intramedullary nail with improved structural properties and compression capabilities, as well as an easy and effective insertion technique and system thereof for stabilizing bones with a intramedullary nail providing dynamic compression throughout the bone healing process.